Anti-reflective (AR) coatings have plenty of practical applications, including glare for reducing eyelids, computer monitors and your smartphone's screen when outdoors. Now, researchers in Penn State have developed an AR layer that improves existing coatings to the extent that it can make transparent plastics, such as plexiglass, virtually invisible.
"This discovery came as we tried to make solar panels with greater efficiency," said Chris Gibink, associate professor of electrical engineering, Penn State. "Our approach included concentrating light on small, high-efficient solar cells using plastic lenses, and we had to minimize their loss of reflection."
We needed anti-reflection coating that worked well throughout the entire solar spectrum and in several corners when the sun crossed the sky. They also needed a layer that could stand in time during long periods of outdoor use.
"We would like to find a solution outside the shelf, but there was not one who fulfilled our performance requirements," he said. "So we started looking for our own solution".
It was a high order. Although it is relatively easy to make a layer that will eliminate the reflection of a certain wavelength or in a particular direction, there was no one that could fit all of their criteria. For example, coatings of the orthopedic AR are directed to the narrow visible part of the spectrum. But the solar spectrum is about five times wider than the visible spectrum, so such a layer will not function well for a concentrating solar cell system.
Reflections occur when light travels from one medium, such as the air, into a second medium, in this case plastic. If the difference in their refractive index, which determines how fast the light travels in a particular material, is large air has a refractive index of 1 and 1.5 plastic – then there will be a lot of thinking. The lowest index for natural coating material like magnesium fluoride or Teflon is about 1.3. The refractive index can be evaluated – slowly changing – between 1.3 and 1.5 by mixing different materials, but the gap between 1.3 and 1 remains.
In a paper recently published on the Internet before printing in the magazine Nano letters, Giebink and coauthors describe a new process for bridging the gap between Teflon and air. They used a sacrificial molecule to create pores of nanoscope in the evaporated Teflon, creating a graduated Teflon-air film that was fogging to see a smooth transition from 1 to 1.5, eliminating essentially all reflections.
"The interesting thing about Teflon, which is a polymer, is when you heat it at critical times, the big polymer chains fit in smaller fragments that are small enough to equalize and send steam. When these lands are on the ground, they can refolymerize and to form Teflon, "said Gibink.
When the sacrificial molecules are added to the flux, Teflon will be reformed around the molecules. The dissolution of the sacrificial molecules comes out of a nanoporous film that can be judged by adding more pores.
"We are interacting with a number of companies that are looking for improved plastic antistatic coatings, and some applications are surprising," he said. "They range from eliminating the flare from the plastic cupolas that protect the security cameras to eliminate the reflections inside the virtual / enlarged speakers."
One unexpected application is in high-altitude or unmanned aerial vehicles. These are giant wings that are coated with solar cells. Used primarily for reconnaissance, these planes rely on sunlight to stay near the everlasting flight and so much of the light they receive is at an angle of view where reflections are the highest. One of the companies that makes these solar cells is to explore the AR wrap to see if it can improve the amount of light collected by the UAV.
Because technology is compatible with current production techniques, Giebink believes that coating technology is scalable and widely applicable. At this point, his testing samples stood centrally in Pennsylvania for two years, with a slight change in properties. In addition, the layer is also anti-evasion.
"The coat is well attached to different types of plastic, but not glass," he said. "So it will not be useful for your typical solar panel with a protective glass cover. But if the concentration of photovoltaics returns, a critical part of it is the plastic freshen lens and we can make a difference there."
Transferable coatings of the next generation can improve the efficiency of the solar photovoltaic cell
Baoming Wang et al., Graded index fluoropolymer antifreeze coatings for invisible plastic optics, Nano letters (2019). DOI: 10.1021 / acs.nanolett.8b03886